Concerns over the stability and security of the fossil fuel supplies as well as their environmental impact have generated interest in hydrogen as a sustainable energy carrier. Developing effective hydrogen storage for automotive applications is a central challenge in realizing a hydrogen energy economy. Light metal hydrides possess high storage capacities, but sluggish hydrogen uptake and release kinetics hinder their practical use. Significant improvements in the kinetics of these materials have been achieved with catalytic additives in conjunction with nano-structuring via high-energy ball-milling or by infiltration into mesoporous scaffolds. Although the hydrogen performance of these materials has been extensively studied, the characterization of the morphology, microstructure, and catalyst dispersion lagged behind, hindering progress in system development. The purpose of this dissertation was to provide a deeper understanding of system functionality in nano-confined and ball-milled light metal hydride hydrogen storage materials through direct characterization of the material structure using electron microscopy, spectroscopy, diffraction, and tomography, and utilizing in situ heating to probe the evolution of the materials during hydrogen cycling.
The nano-confinement of LiBH4 in highly ordered mesoporous carbon scaffolds significantly reduces the high pressures and temperatures needed for de/rehydrogenation, but exhibits reduced practical loading limits and a degradation of hydrogen storage capacity with cycling. Examination of the scaffold structure revealed that they grow in domains of different column orientations rather than as single-orientation monoliths. The inaccessibility of some of these domains to infiltration may explain the limited filling capacity and impact other aspects of their performance. It was discovered that the desorption product LiH is ejected during dehydrogenation at 200 °C, forming a granular crust of nano-cubes and cuboids on the outer scaffold surface. These nano-crystals could also migrate completely away from the scaffolds. This ejection of LiH and thus preferential segregation of lithium from boron explains the performance degradation observed in these systems.
Transmission electron microscopy and X-ray energy-dispersive spectroscopy were used to compare the size, morphology, and dispersion of Ni catalyst particles in MgH2 as a function of high-energy ball-milling duration. Electron tomography was applied to determine the three-dimensional catalyst dispersion. It was found that the Ni catalyst is transformed into the intermetallic compound Mg2NiH4 with increasing milling duration, providing an explanation for the higher desorption temperatures with longer milling times. The dissolution of the Ni catalyst particles and conversion into Mg2Ni/Mg2NiH4 continued during dehydrogenation. These results demonstrate that the as-synthesized structure of the Mg-Ni material is not stable and continues to evolve with cycling and explains the degradation in sorption kinetics with cycling reported in this system. Nanocrystalline MgO layers up to a few nanometers thick on the exterior surfaces were observed to fully enclose the MgH2 particle aggregates. During in situ heating, the MgH2 receded from the oxide layer, resulting in hollow or partially hollow oxide shells surrounding coalesced Mg cores. This provides a conclusive answer to the origins of these shell-like structures, which had been reported but not explained.
This work also demonstrated that the form in which the catalyst is introduced during ball-milling can be used as a tool for tailoring the morphology and dispersion of the catalyst. A dramatic reduction in the Ni particle size by up to two orders of magnitude, with an accompanying increase in size uniformity, was achieved for 1-hour-milled material when anhydrous NiCl2 was used as an alternative catalyst to pure Ni nanopowder. Electron tomography revealed that the dispersion of the catalyst was also significantly affected: the Ni formed patches that were limited to the exterior surfaces of the MgH2.
High-energy high-pressure ball-milled TiH2-doped MgH2, for which excellent cyclic stability was reported, was also examined. In this work it was determined that the TiH2 catalyst particles are highly dispersed throughout the MgH2, and that the particles remain both intact and in place during hydrogen desorption, indicating that the material retains the initially synthesized structure and catalyst dispersion with hydrogen cycling. This distribution of catalyst increases the amount of MgH2 in contact with a catalyst particle, increasing the number of nucleation and growth sites for the Mg/MgH2 phases, and may also act as hydrogen gateways to facilitate the transport of hydrogen to the interior material, helping explain the exceptional sorption behavior of this system. Structural differences in the oxide layers of specimens milled for different durations and their behavior during dehydrogenation observed by in situ heating in the electron microscope may explain the worsening desorption performance reported for specimens milled beyond 4 hours.